The field of the disclosure relates generally to turbomachines and, more particularly, to methods and apparatus for pressurizing a fluid within turbomachines.
At least some known turbomachines operate in closed-loop systems, e.g., Brayton power cycles using working fluids including carbon dioxide (CO2) at supercritical pressures and temperatures (sCO2). Such known turbomachines typically exhibit enhanced thermodynamic efficiency benefits over conventional cycles with more conventional working fluids. These known turbomachines include a main compressor, a bypass compressor, and an expansion turbine rotatably coupled to a common rotor shaft and at least one recuperator coupled in flow communication with the main compressor, the bypass compressor, and the expansion turbine. This recompression cycle configuration for high temperature and high efficiency applications splits a fluid compression stream into two parallel paths with similar inlet and exit pressures, but the streams and the associated compressors operate at different temperatures. That is, the main compressor operates at a lower temperature and the bypass compressor operates at a higher temperature.
However, when compressing CO2 to high pressures, i.e., typically in the range from and including 20 Megapascals (MPa) (2900 pounds per square inch (psi)) to and including 30 MPa (4351 psi), the density of the fluid may result in friction, i.e., windage losses on the rotating surfaces of the impellers of the compressors are induced. More specifically, compressor impellers in dense fluids such as sCO2 experience high windage losses due to friction induced on the rotating back faces of the impellers. These losses are detrimental to the efficiency of the overall power cycle of the turbomachine and may be physically detrimental to the compressor impellers.